Verified answer

import tkinter as tk

def first():

   firstButton["bg"] = "#0000ff"

   secondButton["bg"] = "#fff000"

def second():

   firstButton["fg"] = "#fff000"

   secondButton["fg"] = "#0000ff"

root = tk.Tk()

root.title("Україна")

firstButton = tk.Button(root, text="Моя країна", width=20, height=3,

     font=("Arial", 25), command=first)

secondButton = tk.Button(root, text="Україна", width=20, height=3,

     font=("Arial", 25), command=second)

firstButton.pack()

secondButton.pack()

root.mainloop()